A chart turns numbers into a picture so people can compare them or notice a pattern. It is like marking water levels on a wall: the marks make change easier to see than a list of measurements.
A bar chart might compare how many items were sold each month. A line chart could show whether a value has risen or fallen over time.
Label what the numbers mean and do not rely on color alone. Give the important data in text or a table too, so people can understand the chart even if they cannot see it.